Semiconductor fins are widely used in advanced semiconductor manufacturing technology because of the increased integration levels they provide. For example, when a transistor device such as a FinFET, fin Field Effect Transistor, is formed, it provides a gate width that may be larger, by an order of magnitude, than the gate width of a conventional transistor formed on the same substrate region over which the semiconductor fin is formed because the gate extends over the top and sides of the fins, all of which serve as channels. Semiconductor fins have high aspect ratios and require advanced and dedicated processing operations to forming working FinFET or other devices using the semiconductor fins.
When these devices are formed using advanced processing technology and miniaturized features of increasingly small dimensions, particular care must be taken in forming and positioning the features. In the semiconductor fabrication industry, transistors have typically been formed utilizing polysilicon gate electrodes. Polysilicon has been preferred because it is thermally robust and has other special and desirable characteristics. Polysilicon gates can withstand the processing of other elements of the transistors such as the source and drain regions, during dopant drive-in or other high temperature processes such as annealing processes. From a device standpoint, however, the use of polysilicon as the gate material is less advantageous than the use of metal as the gate material. A polysilicon gate is subject to the formation of a depletion region in operation in which charge carriers are depleted from the polysilicon material above the gate dielectric. A metal electrode provides the advantage that charge carriers are not depleted but remain plentiful throughout the gate electrode.
Metal gates are also much better conductors of electricity resulting in reduced gate contact resistance which provides faster device performance. The manufacture of metal gates, however, may pose serious challenges. For one, metal gates are not thermally robust like polysilicon and therefore cannot be exposed to high temperatures during processing of transistors or other elements of integrated circuits (ICs).
In recent years, there has been an effort to overcome the limitations of metal gate processing and the operational deficiencies of polysilicon gates through a process of forming transistor structures initially having polysilicon gates which are better able to withstand the more severe processing conditions of the immediately subsequent processing operations. Thereafter, in latter stages of processing when processing conditions are less severe, the polysilicon gates are removed from the structures and replaced with permanent metal gates. This replacement gate process, as it is known, also carries with it limitations that become an even greater concern as technologies advance, integration levels increase and feature sizes become smaller. This is especially true in the formation of FinFET devices and other transistors formed over semiconductor fins. For example, aluminum is a metal that is advantageously used as a metal gate material for a number of reasons including ease of processing using damascene planarization technology. To inhibit aluminum diffusion that adversely impacts device performance, however, a blocking layer must be used along with the aluminum metal gate material. This is also true for other suitable metals. The blocking layer must be compatible with the eWF (electrical Work Function) of the metal layer used and must be formed using a deposition process that will enable the film to be formed conformally within openings that may have high aspect ratios so that the metal layer can subsequently be deposited in the openings. According to conventional technologies, thermal atomic layer deposition, ALD, may be used to form the blocking layer. When thermal ALD is used to form very thin films, however, the films typically include pitting which is a defect that can adversely affect or even destroy device functionality.
An improved method is therefore needed that enables the formation of replacement metal gates and which addresses the above shortcomings conventionally associated with replacement metal gate formation processes.